Generally, ceramic wiring substrates are produced by separating a multi-piece ceramic wiring substrate array into individual pieces along dividing grooves provided on the front surface or back surface of the wiring substrate array. In connection therewith, there has been proposed a method for producing a multi-piece wiring substrate array, in which a blade having a specific edge angle is pressed against a green sheet laminate at positions where dividing grooves are to be formed, so that chips or burrs are less likely to be generated at a metal layer, etc. located in the vicinity of the thus-formed dividing grooves during separation of the substrate array (see, for example, Patent Document 1).
However, when dividing grooves are formed in a green sheet by means of a blade through the production method described in Patent Document 1, since a fracture surface (cracking) meanders at the inner wall of a through hole or bottomed hole which is to become a notch and which crosses with the thus-formed dividing grooves in a radial direction of the hole, burrs or cracks are likely to be generated. In addition, since the depth of a dividing groove formed by means of the aforementioned blade must be smaller than that of the position of a plating wire for connecting the internal wirings of two adjacent wiring substrate units, the dividing groove must be formed to be shallower than the bottom of a bottomed hole or shallower than a stepped portion of a through hole having two concentric portions of different inner diameters, which results in easy generation of burrs, etc.
Also, when a metal conductor layer is formed on the inner wall of the through hole, burrs are likely to be generated through breakage of the metal layer, and a cut surface of the non-plated conductor layer is exposed after separation of the substrate array. Therefore, the conductor layer may exhibit poor reliability during soldering for mounting of a component on the conductor layer.
Meanwhile, there has been proposed a multi-piece wiring substrate array, in which a dividing groove having different, discontinuous depths is formed by means of a special blade having a single base and different edge heights (widths), so as to prevent burrs or cracks, which would otherwise be generated in association with a meandering fracture surface (cracking) at the inner wall of a through hole in a green sheet (see, for example, Patent Document 2).
However, in the case of the multi-piece wiring substrate array described in Patent Document 2, every time a through-hole-forming position is changed to any position which crosses with a dividing groove, there must be provided a number of special blades having different edge heights (widths) and lengths corresponding to both a portion in the vicinity of the surface of a green sheet and a portion in the vicinity of a through hole. Thus, the multi-piece wiring substrate array poses a problem in that various types of special blades are required, which results in considerably high production cost and low productivity.